This invention relates to an improvement on an amplifier and more particularly to an amplifier adapted to drive an actuator used with an autofocusing mechanism or autotracking mechanism of an optical pickup loaded on, for example, an optical digital audio disc (DAD) player.
Recently, a technique of a digital audio disc (hereinafter referred to as "DAD") has been developed, making it possible to reproduce higher quality sounds than those derived from the conventional analog type. The DAD player which draws out analog audio signals from the DAD scans a plurality of pit tracks spirally formed on the DAD plane by means of, for example, a laser pickup.
As is well known, the plural pit tracks are scanned by various servo mechanisms such as an autofocusing mechanism and an autotracking mechanism.
The autofocusing mechanism is designed to detect the attainment or failure of focusing by, for example, a focusing error detector. A signal denoting errors caused by the focusing error detector is electrically amplified by the drive circuit of a focusing control actuator. Focusing is always assured by controlling the position of an object lens in the laser pickup to be shifted vertical to the DAD plane.
The autotracking mechanism is designed to let a tracking error detector detect the tracking condition. A tracking error output signal from the tracking error detector is amplified by the drive circuit of a tracking control actuator. The best tracking condition is always assured by controlling the laser reflector in the laser pickup to be radial to the DAD plane by means of the actuator.
The actuators for controlling the focusing and tracking generally have input/output transfer characteristics of approximately -12 db/oct. Therefore, the higher the frequency of an input signal, the larger the input power required for the actuator.
The output level of an error output signal from an error detector such as a focusing error detector or tracking error detector generally rises with the frequency of the error signal. Consequently, an amplifier of the error signal, for example, an actuator-driving amplifier must have a sufficiently broad dynamic range in a high frequency range. The DAD is sometimes affected by momentarily large external vibrations. To satisfactorily cope with such event, too, the actuator-driving amplifier should have a sufficiently broad dynamic range.
The conventional amplifier applied to the above-mentioned object involves a differential amplifier as a drive section and an SEPP (Single Ended Push-Pull) amplifier consisting of complementary transistors as a power amplifier section. This arrangement assures the issue of an output signal from the power amplifier section to drive the desired load, for example, the above-mentioned actuator. Positive and negative power input terminals of the differential amplifier of the driving section are respectively connected to positive and negative power sources. Also collectors of the complementary transistors of the power amplifier section are respectively connected to the positive and negative power sources. In this case, an error input signal is supplied to an inverted input terminal of the differential amplifier. An output signal from the power amplifier section is delivered to the load and also fed back as a negative feedback signal to a noninverted input terminal of the differential amplifier.
With the conventional actuator driving amplifier of the above-mentioned arrangement, it was necessary in an abnormal case calling for high amplification to provide a sufficiently higher level of positive and negative voltage than in the normal condition when low amplification is carried out in order to assure a sufficiently broad dynamic range. Therefore, such conventional amplifiers which include an SEPP amplifier in the power amplifier section consumed objectionably large power as a whole.
Another known amplifier is the type which is constructed by inserting a parallel circuit consisting of a resistor and capacitor in between the power amplifier section and the load. This amplifier is the type which is intended to protect the load by restricting the flow of a highly amplified signal into the load through a resistor in a low frequency region. A capacitor compensates for a dynamic range in a high frequency region. With respect to the power consumption efficiency, however, the other known amplifier is accompanied with the same drawback as the aforesaid conventional amplifier.